Buying a large format scanner for automatic raster to vector conversion (Part I)

(updated April 2011)

Three scanners suitable for raster to vector conversion

Three large format CIS
scanners suitable for
scanning drawings for
raster to vector conversion:
The Colortrac Ci 40 (top),
the Contex SD Series
(middle) and the Graphtec
CSX300 (bottom).

By far the biggest barrier to good automatic vectorization results is poor quality scanned images.

With a good quality scanned image, good results are possible from competent vectorization software. With a poor image, the odds are stacked against a good raster to vector conversion result from the start. GIGO! (Garbage In, Garbage Out).

The chief reasons for poor quality scanned images are that too many scanner operators lack proper training and do not understand what scanned image quality looks like. Because manufacturers and resellers claim scanning is easy, inexperienced temporary employees are often delegated with the job of creating scanned images for archiving and any possible future raster to vector conversion.

So, it follows that if you have access to a large format scanner and know what you are doing and are able to scan the drawing with more appropriate settings you will get a better scanned image and better raster to vector conversion results. Unfortunately, too many people wanting to convert scanned drawings to CAD or CNC receive raster images from other sources and do not have the ability to rescan poor images. In such cases, you will need to choose a raster to vector conversion program which has a good set of raster editing tools. However, no amount of raster image editing and tidy-up can replace the benefits of starting the vectorization process with a good and appropriately scanned image. GIGO!

This article is written for anyone looking for advice about buying a large format scanner to use with an automatic raster to vector conversion program.

Types of Scanner

There are two basic types of scanner application - technical imaging (drawings, blueprints and maps, etc.) - and graphics (artwork, photographs, etc.). Technical imaging is in monchrome or limited color while graphics is in full color.

Catering for these two basic scanner application areas are two types of large format scanner using different optical imaging technology, either CCD (Charge Coupled Device) or CIS (Contact Image Sensor). Their differences affect how they capture data, especially fine line sharpness and color.

It is generally agreed by experts that CIS scanners provide sharper results with fine line detail in technical drawings while CCD scanners capture a wider range of colors more accurately, making them the best solution in demanding color graphics applications. This is not a hard and fast distinction but it does allow AEC, CAD and GIS users wanting to vectorize technical images and maps to identify the generally more affordable CIS wide format scanners as their best option.

Scanner Makes

There are three main large format scanner manufacturers - Colortrac, Contex and Graphtec.

Contex is the biggest large format scanner manufacturer and sells its imaging products through distributors and resellers and also via OEMs like HP, Oce and Vidar who put their names on Contex devices.

Other manufacturers of large format scanners include Image Access, KIP, Ricoh and Xerox. Generally speaking, any scanner supplier not mentioned above is an OEM for one or other product. Several MFP (multi-function peripheral) scan and copy stations OEM Graphtec's entry-level CIS scanner.

CIS vs CCD

Graphtec was first to introduce a CIS scanner, followed by Colortrac and most recently by Contex. Graphtec do not manufacture CCD scanners. As a result, Graphtec claim that CIS optical imaging technology is suitable for high quality graphics capture.

Colortrac and Contex manufacture both CIS and CCD wide format scanners. Both agree that CCD suits high fidelity, full color graphics capture more than CIS which they again agree is best suited to technical drawings and maps in monochrome and limited color. CIS is suitable for the majority of AEC, CAD and GIS color capture applications.

So, for the purposes of converting scanned technical images and maps into a vector format for editing in CAD, you should first be looking at buying a CIS scanner. However, if you have a requirement to capture high quality graphics images as well, a CCD scanner need not be ruled out. A CCD scanner is more than capable of providing results suitable for automatic raster to vector conversion if properly used.

A CCD scanner is better suited to scanning technical images than a CIS scanner is suited to capturing full color graphics.

Having determined the type of scanner you believe best suits your large format scanning needs, the next thing you need to do is cut a swathe through the manufacturers' claims for their products. The two areas most clouded in bull and misrepresentation are Resolution and Speed.

We will look at these in Part II.